Cladding-pumped waveguide optical gyroscope

ABSTRACT

A waveguide optical gyroscope (WOG) is disclosed, which may include: an emitter; an integrated interferometer disposed on a silica planar lightwave circuit (PLC) and comprising a multilayer waveguide loop disposed in a first cladding material and interposed between layers of at least a second cladding material having an index of refraction lower than an index of refraction of the first cladding material; a pump source configured to pump the first cladding material with a signal that compensates for a propagation loss in the multilayer waveguide loop; and a micro-optic component configured to receive an output of the emitter and to guide the output into the integrated interferometer.

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.15/408,074, filed Jan. 17, 2017, which claims the benefit of U.S.Provisional Patent Application No. 62/398,255, filed Sep. 22, 2016, allof which are incorporated by reference in their entirety.

FIELD

This application is generally related to inertial navigation devicesincluding optical gyroscopes.

BACKGROUND

Optical gyroscopes are important for inertial navigation. Opticalgyroscopes are expected to play more important roles for applicationswhere GPS is denied or compromised.

The Sagnac effect is directly related to optical gyroscopes. Accordingto the Sagnac effect, the phase or frequency of light circulating in aloop geometry is modified by the rotation of the loop. The effect ismanifested by a change in interference between the two counterpropagating lights in the loop.

Optical gyroscopes can be broadly categorized into the interferometrictype where the phase shift is measured, and the resonant type where afrequency shift due to rotation is measured. In both cases, thesensitivity of the gyroscope is linearly proportional to the length ofthe loop.

Chip-scale optical gyroscopes have been advanced to reduce the size ofthe device to adopt optical gyroscope technologies to other applicationareas. However, the optical propagation loss of typical waveguides isgreater than 1 dB/m. This is several orders of magnitude inferior tooptical fiber loss of about 0.2 dB/km. As a result, the loop length, andin turn the sensitivity, of the waveguide optical gyroscope is limited.

SUMMARY

The foregoing needs are met, to a great extent, by a waveguide opticalgyroscope (WOG) utilizing a rare-earth doped waveguide core and claddingpumping. In so doing, a loss-free and symmetric gyroscope loop isobtained.

There has thus been outlined, rather broadly, certain embodiments of theinvention in order that the detailed description thereof herein may bebetter understood, and in order that the present contribution to the artmay be better appreciated. There are, of course, additional embodimentsof the invention that will be described below and which will form thesubject matter of the claims appended hereto.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to facilitate a fuller understanding of the invention,reference is now made to the accompanying drawings, in which likeelements are referenced with like numerals. These drawings should not beconstrued as limiting the invention and intended only to beillustrative.

FIG. 1 illustrates a cross-sectional view of a waveguide layer of a WOGaccording to an aspect of the application.

FIG. 2 illustrates a configuration of a cladding-pumped WOG according toan aspect of the application.

FIG. 3 illustrates a WOG according to an aspect of the application.

FIG. 4 illustrates a multi-layer Sagnac loop with cladding pumpingaccording to an aspect of the application.

FIG. 5 illustrates a cross-sectional view of the cladding pumpedwaveguide loop along a dashed line of FIG. 3.

DETAILED DESCRIPTION

In this respect, before explaining at least one embodiment of theinvention in detail, it is to be understood that the invention is notlimited in its application to the details of construction and to thearrangements of the components set forth in the following description orillustrated in the drawings. The invention is capable of embodiments orembodiments in addition to those described and of being practiced andcarried out in various ways. Also, it is to be understood that thephraseology and terminology employed herein, as well as the abstract,are for the purpose of description and should not be regarded aslimiting.

Reference in this application to “one embodiment,” “an embodiment,” “oneor more embodiments,” or the like means that a particular feature,structure, or characteristic described in connection with the embodimentis included in at least one embodiment of the disclosure. Theappearances of, for example, the phrases “an embodiment” in variousplaces in the specification are not necessarily all referring to thesame embodiment, nor are separate or alternative embodiments mutuallyexclusive of other embodiments. Moreover, various features are describedwhich may be exhibited by some embodiments and not by the other.Similarly, various requirements are described which may be requirementsfor some embodiments but not by other embodiments.

In one aspect of the application, interferometric waveguide opticalgyroscope (I-WOG) sensors are described. The I-WOG sensors may be usefulfor munitions navigation in the absence of GPS. One feature of the I-WOGsensors is a three dimensional (3-D) integrated Sagnac interferometerimplemented with a rare-earth (RE)-doped silica waveguide on silicon.The RE-doped waveguide is illuminated by an athermal pump laser. Thepump laser light is guided in and through the cladding layer to providecompensation for the loss of the signal guide by the waveguide core. Thelight enables an optical path longer than 120 m necessary for achievingnavigation-grade inertial guidance. A similar RE-doped waveguidestructure, with pump laser light directly injected to the waveguide coreprovides a signal input with wavelength stability better than 1 ppm.This helps obtain the scale factor stability under a severe operatingenvironment.

A WOG has no moving parts and therefore possesses distinct advantagesover MEMS-based inertial sensors in providing capability for operatingthrough the harsh shock and vibration stresses expected during launchconditions. Silica PLCs have excellent mechanical properties comparableto silicon-based electronics historically shown to be survivable up to50,000 g. Accordingly it is envisaged that a waveguide implementation ofan interferometric FOG can produce a navigation-grade sensor. Thetechniques compensate for the low loss of 2-3 dB/m by weakly pumpingactive RE-doped waveguides to achieve completely lossless propagation.This can also provide overall signal gain. The doped waveguide alsoprovides means for implementing an on-chip light source with muchreduced environmental sensitivity.

According to another aspect of the application, hybrid integration usinga silica planar lightwave circuit (PLC) on silicon is provided forintegrating the necessary optical components on chip. This includes amicro-optic isolator and phase modulators with sufficient mechanicalstrength and low insertion losses.

WOG

FIG. 1 illustrates a waveguide optical gyroscope (WOG) 100 in accordancewith aspects of the present disclosure. The WOG 100 may comprise awaveguide 102 (e.g., rare-earth (RE) doped waveguide) and dual cladlayers 104 and 106. The inner clad layer 106 may be pumped usingcladding pumping, as described herein. As an example, the WOG 100 may beconfigured as a loss-free loop, such as a loss compensated loop. Asshown, the waveguide 102 may be embedded in a high-index (n₁) cladding106 that is sandwiched between lower-index (n₂<n₁) cladding layers 104.The WOG 100 may further comprise a silicon substrate 108. As such, apump light 200 (e.g., laser) may irradiate in the plane of thehigh-index cladding layer 106 and the irradiated energy may pump thewaveguide 102, as shown in FIG. 2.

As shown in FIG. 2, the waveguide 102 may be configured in a loopconfiguration and coupled to one or more of a signal source 202 (e.g.,ASE source) and a detector 204 (e.g., photodetector). It is to beunderstood that the pump light 200 may be propagated in a manner suchthat the reciprocity of the clockwise (CW) and counter-clockwise (CCW)rotation of a signal beam received from the signal source 202 is notviolated. The sensitivity of the WOG 100 may be dependent on thesymmetry between the CW and CCW propagation in the loop configuration ofthe waveguide 100. Near uniform illumination of the waveguide 100 by thecladding pumping facilitates nominal to no longitudinal gain/lossvariation along the waveguide 100 and maintains the symmetry between theCW and CCW propagation.

FIG. 3 illustrates an interferometric waveguide optical gyroscope(I-WOG) 300 in accordance with aspects of the present disclosure. TheI-WOG 300 may include an amplified spontaneous emission (ASE) source302, one or more phase modulators 304 (e.g., for biasing), a Sagnacinterferometer 306 or sensor, and a photodetector 308. Other componentsmay be included and configured in various manners such as isolators andpolarizers, for example. As an example, the I-WOG 300 may be configuredas a loss-compensated Sagnac sensor using cladding-pumped RE-dopedwaveguide is described. As a further example, one or more components ofthe I-WOG 300 may be disposed on a planar lightwave circuit (PLC) suchas hybrid-integrated on a silica PLC 318

The ASE source 302 may include a waveguide 301 and a pump source 303configured to energize the waveguide 301. The pump source 303 of the ASEsource 302 may be or comprise an athermal pump laser. The waveguide 301may comprise a rare-earth (RE) doped silica.

The Sagnac interferometer 306 (e.g., sensor) may include a waveguideloop 321 disposed in a first cladding material interposed between layersof at least a second cladding material having an index of refractionlower than an index of refraction of the first cladding material. As anexample, the waveguide loop 321 comprises a rare-earth (RE) dopedsilica. The Sagnac interferometer 306 may be configured to receive anoutput signal of the ASE source 302. As an example, the output of theASE source 302 may pass through one or more of a micro-optic isolator314, a thin-film optical filter (not shown) a micro-optic polarizer 316,and the phase modulators 304 before guided into the Sagnacinterferometer 306. A pump source 320 may be configured to pump at leastthe first cladding material of the Sagnac interferometer 306 with anin-plane pump signal. As such, the propagation loss in the Sagnacinterferometer 306 may be compensated by the pump source 320. The pumpsource 320 may be or comprise an athermal pump laser. In certainaspects, the Sagnac interferometer 306 may include a trench 322 formedalong a transverse axis of the multilayers of cladding material andspaced from the pump source 320 such that the waveguide loop is disposedbetween the pump source and the trench 322. A reflective coating, suchas metal, may be disposed in the trench 322 to reflect the pump lightback into the cladding layer, effectively confining the pump light. Pumprays may be fully absorbed by the RE-doped waveguide core, while theytraverse the waveguide loop that is optically confined by the reflectiveside walls surrounding 321.

As an illustrative example, the ASE source 302 may include an athermalpump laser 310 and a ˜2-m long RE-doped waveguide 312. A micro-opticisolator 314 and thin-film optical filter may be configured to lock theASE wavelength. A high polarization extinction (>40 dB) micro-opticpolarizer 316 may be configured to control the polarization beforetransmitting the light into the Sagnac interferometer 306. For phasemodulation, an approximate 1.5 cm X-cut lithium niobate phase modulators304 may be hybrid-integrated to the silica PLC 318. The Sagnacinterferometer 306 may include active loops of multi-layer waveguidespumped by two pumps 320 (e.g., 980-nm pump signals), facing in oppositedirections, located at/near the center of the active loop. The outer andinner perimeters of the loops may be surrounded by trenches 322configured with metal coating to operate as reflectors and therebyserving to confine the pump beam to improve the pump efficiency. A COTSInGaAs photodetector 308 (˜10 MHz bandwidth) is hybrid-integrated fordetecting the gyro signal.

As provided in the exemplary embodiment as illustrated in FIG. 4, athree dimensional (3-D) integrated waveguide 402 may be configured afiber loop in an interferometric fiber-optic gyroscope (I-FOG) 400. TheI-FOG 400 of FIG. 4 may be configured as the Sagnac interferometer 306of FIG. 3. The I-FOG 400 may be configured with extremely lowpropagation loss (e.g., 0.2 dB/km) of silica fiber at or near 1550 nm.It is envisaged in this application that achieving chip-scale longoptical delays may leverage a low-loss (2 dB/m) silica waveguide (e.g.,waveguide 402) disposed on silicon (e.g., with index contrast ˜1.5-2%).A tight bend radius (e.g., about 1-2 mm) of the waveguide loop 403 andsmall confined mode size (˜5 μm) facilitates high density packing ofwaveguides with negligible (−60 to −70 dB) overlap between the modes ofneighboring waveguides (15-20 μm spacing). As an example, up to a 30-mindelay is implemented in a single waveguide layer. Waveguide loss may beovercome by doping the waveguide 402 with Er or Er/Yb and pumping thewaveguide 402 with ˜980-nm pump light.

In reverence the FIG. 4, a pump beam 404 may be side-coupled anduniformly distributed in a cladding 406 surrounding at least a portionof the waveguide 402 interposed between cladding layers 408 configuredto maintain the symmetry of the waveguide 402. Minimizing the backgroundloss of the waveguide 402 may facilitate reduction of the pump powerrequired to compensate for the waveguide loss. As an example, RE-dopedwaveguides may exhibit background loss lower than 3 dB/m. It iscalculated that about 60 mW of 980 nm pump light (e.g., pump beam 404)may be absorbed in 120 m of an Er-doped (Er concentration ˜10²⁵/m³)waveguide to generate sufficient optical gain for compensating thebackground waveguide loss. The pump beam 404 may be sourced from 980-nmpump lasers operated athermally over the entire relevant temperaturerange by hybrid integrating the laser and an athermal micro-opticvolume-grating wavelength locker (Δλ=0.01 nm/° C.) onto a silica PLC.

3D Photonic Integration

In another embodiment, the waveguide 402 may comprise a multi-stackdesign to implement Sagnac sensors having multiple waveguide layers.Such a configuration multiplies the length of the delay to longer than120 m in order to achieve a shot-noise limited bias stability of lessthan 0.01 degree/hr. The multi-layer design may have no waveguidecrossings which could contribute to detrimental back reflection andexcess loss, nor in-plane or inter-plane modal overlap creating crosstalk, except at the inter-layer transition region. The mode size may beexpanded from ˜5 μm to larger than 10 μm only locally in the transitionarea using a mode-size expander (FIG. 5), for example. This controlledtransition between the layers without waveguide/mode crossings will beutilized to optimize the loop-winding geometry to minimize the impact ofnon-reciprocal perturbations.

The waveguide 402 in the Sagnac loop may include a RE-doped silicawaveguide core (Δn˜2.5%) and may be surrounded by cladding (Δn˜0.5%)layers 406, 408 for guiding the pump light 404. The refractive indicesare measured relative to that of the cladding layer 408 (e.g., bufferglass layer (n₀)) between the cladding 406 and a silicon substrate 401.The index of refraction of the waveguide 402 may be controlled fromabout 0.5% up to 4% by adjusting the concentration of glass ingredients,such as P and Ge, as well as the active ions such as Er and Yb. Thewaveguide 402 spacing (˜15 μm) is sufficiently larger than the opticalmode size such that unwanted power cross talks between the neighboringwaveguides will be negligible

Athermal RE-Doped Waveguide Light Source

One of the challenges in I-FOGs is scale-factor stability. This may becompounded in harsh environments due to extreme temperature cycling,shock and vibration. In view of this, the RE-doped waveguide technologyand hybrid-integrated athermal 980-nm pump laser described in thisdisclosure generates amplified spontaneous emission (ASE) from the dopedwaveguide. For efficient ASE generation, pump light is injected into thecore of the doped waveguide. The mode expander on the doped waveguideside and active alignment in flip-chip bonding of the athermal pumplaser will minimize the insertion loss. A thin-film optical filter whosepassband (1540-1560 nm) overlaps with the spectrally flat part of ASEfrom Er ions will be integrated in the micro-optic isolator package,which will be flip-chip bonded to the silica PLC. Optimizing thethin-film optical filter helps achieve the required wavelength band andtemperature stability.

According to another embodiment, a hybrid integration process similar tothat used for the cladding pumps may be used to integrate an athermalpump laser to core-pump a ˜2 m doped, tightly coiled, doped waveguide. Amicro-optic isolator package (˜2 mm size) includes a double-stageisolator (40-dB isolation) and collimating lens coated with opticalbandpass filter (1540-1560 nm) to stabilize the central wavelength ofthe ASE irrespective of the temperature. Approximately 4-mW ASEgeneration per polarization within the band is expected using about 250mW electrical power. Insertion loss of the isolator is minimized byusing waveguide spot-size converter on the silica PLC.

Cladding-Pumping with Athermal Pump Lasers

According to another aspect of the application, a cladding pumpingscheme may be used to achieve uniform population inversion along thedoped waveguide cores. The Sagnac interferometer 306 (e.g., sensor) isillustrated in FIG. 5, which may be a representation of a cross sectiontaken across line 5-5 in FIG. 3. As shown in FIG. 5, Sagnacinterferometer 306 the may include the waveguide loop 321 disposed in afirst cladding material 323 interposed between layers of at least asecond cladding material 325 having an index of refraction lower than anindex of refraction of the first cladding material 323. As an example,the waveguide loop 321 comprises a rare-earth (RE) doped silica, whichmay maintain the reciprocity of the loop. Efficient pump geometry is maybe used to achieve power consumption management. First, the light fromthe athermal pump source 320 may be coupled to the cladding layers, 323.Vertical alignment may be achieved by flip-chip bonding of the athermalpump source 320 on solder stacks whose thickness is precisely controlledby deposition.

The numerical aperture (NA) of the pump light of the pump source 320 maybe controlled such that the pump light is vertically guided in thecladding layer 323. Second, the pump source 320 may operate athermallywith the output wavelength varying less than 1 nm around the Er/Ybabsorption peak over 100° C. temperature swings. The perimeter of thedoped Sagnac loop area may form a cavity or trench 322 to trap the pumplight. The trench 322 may comprise a metallic mirror coating to form avertical reflecting surface around the waveguide loop 321. The shape ofthe trench 322 may be configured to promote chaotic pump propagation andeven distribution of the pump light throughout the Sagnac loop. Touniformly excite all RE-doped waveguide cores, the trajectory of thepump rays in 306, as viewed from above in FIG. 3, is advantageous to bechaotic so that it ergodically covers the loop area. One problem oftracing rays confined in a reflective two dimensional cavity, formed bythe reflective trench walls, may be similar to the problem of dynamicalbilliards in Newtonian mechanics. It is understood in the art thatcertain geometry, such as a racetrack, promotes chaotic trajectory. Twopump sources 320 may be used for symmetric illumination of the area.With ideal coupling (˜1 dB coupling loss) and negligible potential pumpleakage via scattering at the core waveguide walls, or losses at themirror surfaces, about 80 mW pump power is needed to fully compensatefor 2 dB/m passive propagation loss. Assuming about 40% wall plugefficiency of COTS pump lasers, 200 mW electrical power is needed. Anyreduction in passive waveguide loss results in commensurate reduction inthe pump power requirement.

Hybrid Integration

According to another aspect of the application, the maturity of silicaPLC provides versatile tools for engineering ruggedized athermal I-WOGdevices. For example, the symmetry of the power splitting of a Y-branchor directional coupler can be much more accurately controlled in silicaPLC than in other platforms such as lithium niobate or III-V materials.In addition, mode size engineering can be reliably achieved withnegligible excess loss, which in turn allows facile integration ofvarious micro-optic components, such as isolators and polarizers, whoseperformance far exceed what can be achieved with waveguide equivalents.Various bonding methods, including solder bonds and hardened epoxy, canbe used to achieve ruggedized integration of optical components havingperformance optimized for the task. In addition, local substitution inthe over-clad materials, using polymer or other glasses, can achieveathermal performance in optical delay or power splitting ratio in thecouplers.

In an embodiment, similar mode expanders also facilitate hybridintegration of the remaining components including the micro-opticpolarizer and an array of X-cut lithium niobate (LN) modulators. Again,these components will be solidly bonded to the silica PLC using solderstacks similar to those used for integrating the pump lasers. It isenvisaged that while the power split ratio of a silica Y-branch is quitesymmetric and nearly independent of the temperature, the hybridintegration of lithium niobate modulators may introduce asymmetry. Thiscan be addressed by various trimming techniques such as controlledscoring of the silica waveguide cladding.

LN modulators are subject to phase drift/noise from pyro- andpiezo-electric effects. Good modulator design such as the X-cut geometrywith buffer and shielding layers can significantly reduce thepyroelectric effect to <1 mrad/° C. The pyroelectric phase variation canbe calculated using a temperature monitor or compensated by fast biasadjustments using signal processing, such as the four-phase modulationtechnique. It is expected the effect of the pyroelectric drift of the LNmodulator will be manageable and should not seriously degrade theperformance of the gyro. Piezoelectric phase variations are moredifficult to cope with owing to potentially high-frequency vibration.experience in modeling the effects of vibration on electronic componentsin high-shock environments, which will be used for estimating andmitigating the piezoelectric effects.

High-Bandwidth Gyroscope Operation

Inertial guidance of projectile or UAV that may be subject to rapidrotation may require additional high-bandwidth control; maintaining1-ppm scale factor stability/linearity over the wide dynamic range up to±100,000°/sec in the case of guided munition is extremely challenging.The scale factor stability is directly related to the stability of thekey system elements, specifically the loop area, the source centerwavelength, and the modulator's V_(π). The loop area will vary withtemperature, but it can be calibrated out by directly measuring the looptemperature. Excessive stress on the coil due to centripetal forces maycause small deformations in the shape if not adequately managed in themechanical design. The source wavelength stability will be derived froma thin film optical filter properly packaged so as to ensure negligiblerotation relative to the waveguide. In addition, the ASE source must bevery stable to minimize any spectral power variation over the passbandof the filter. Achieving this stability requires a stable pump laserwith stable coupling into the waveguide.

Achieving both the required sensitivity and supporting ±100,000°/secaxial rotation also means that the gyro must operate over multiplefringes. One option is to directly count the fringes either in open- orclosed-loop mode. Alternately, a lower resolution gyro, or even theaccelerometers, could be used to evaluate which fringe the highresolution gyro is operating on. Closed loop operation will be used fortip and tilt monitoring during and after the launch. However, carefulanalysis is required to determine an optimal mode of operation for theaxial rotation during the launch, given the system constraints.

In an embodiment, a SWAP per axis is: (i) Volume/Weight: ˜0.5 cm³/˜1 g;(ii) Power consumption: <470 mW (cladding pump laser: 200 mW, ASEsource: 250 mW, Phase modulator driver: <20 mW).

While the system and method have been described in terms of what arepresently considered to be specific embodiments, the disclosure need notbe limited to the disclosed embodiments. It is intended to cover variousmodifications and similar arrangements included within the spirit andscope of the claims, the scope of which should be accorded the broadestinterpretation so as to encompass all such modifications and similarstructures.

What is claimed is:
 1. A waveguide optical gyroscope (WOG), comprising:an emitter; an integrated interferometer disposed on a silica planarlightwave circuit (PLC) and comprising a multilayer waveguide loopdisposed in a first cladding material and interposed between layers ofat least a second cladding material having an index of refraction lowerthan an index of refraction of the first cladding material; a pumpsource configured to pump the first cladding material with a signal thatcompensates for a propagation loss in the multilayer waveguide loop; awaveguide spot-size converter (i) further disposed on the same silicaPLC and (ii) configured to reduce an insertion loss of a micro-opticcomponent by an amount satisfying a criterion; a symmetry between aclockwise propagation and a counter-clockwise propagation in themultilayer waveguide loop; and the micro-optic component (i) furtherdisposed on the same silica PLC, (ii) configured to receive an output ofthe emitter and to guide the output into the integrated interferometer,and (iii) comprising a micro-optic isolator and a plurality of phasemodulators, wherein the symmetry is maintained by a substantiallyuniform illumination by the pump source that minimizes longitudinalgain/loss variation along the multilayer waveguide loop.
 2. The WOG ofclaim 1, wherein the multilayer waveguide loop comprises a rare-earth(RE) doped silica.
 3. The WOG of claim 1, wherein the micro-opticisolator is configured to block backward propagating light from themultilayer waveguide loop towards the emitter.
 4. The WOG of claim 1,wherein the micro-optic component includes a micro-optic polarizerconfigured to control polarization of the guided output beforetransmission into the integrated interferometer.
 5. The WOG of claim 1,wherein the phase modulators comprise a plurality of X-cut lithiumniobate phase modulators.
 6. The WOG of claim 1, wherein the integratedinterferometer includes an active loop of multi-layer waveguides pumpedby the pump source and another pump source, the pump sources facingopposite directions and being located at a center of the active loop. 7.The WOG of claim 1, wherein the multilayer waveguide loop is doped withonly Erbium or with Erbium and Ytterbium.
 8. The WOG of claim 1, whereinthe pump signal is side-coupled and uniformly distributed in the firstcladding material surrounding at least a portion of the multilayerwaveguide loop.
 9. The WOG of claim 1, wherein the pump signal issourced from pump lasers operated athermally.
 10. The WOG of claim 1,wherein the multilayer waveguide loop comprises multiple stacks toimplement a Sagnac sensor having multiple waveguide layers and tomultiply a length of delay to control a shot-noise limited biasstability.
 11. The WOG of claim 10, wherein the multiple stacks have nowaveguide crossings, except at an inter-layer transition region, toreduce an impact of non-reciprocal perturbations.
 12. The WOG of claim1, wherein a spacing of the multilayer waveguide loop is larger than anoptical mode size to reduce power cross talk between neighboringwaveguides.
 13. The WOG of claim 1, further comprising: an opticalfilter having a passband that overlaps with a spectrally flat part ofthe output.
 14. The WOG of claim 13, wherein the micro-optic componentincludes a double-stage isolator and a collimating lens that is coatedwith the optical filter and that operates irrespective of a temperature.15. The WOG of claim 1, further comprising: cladding pumping configuredto achieve uniform population inversion along a plurality of dopedwaveguide cores.
 16. The WOG of claim 1, further comprising: a trenchconfigured to promote chaotic pump propagation throughout the multilayerwaveguide loop.
 17. The WOG of claim 16, wherein the trench is furtherconfigured to promote even distribution of the signal throughout themultilayer waveguide loop.
 18. The WOG of claim 1, further comprising: alocal substitution in over-clad materials using a polymer to achieveathermal performance in an optical delay or a power splitting ratio incouplers.
 19. The WOG of claim 1, wherein the WOG operates over multiplefringes, the fringes being counted either in an open- or closed-loopmode.
 20. The WOG of claim 1, further comprising: a mode-size expanderon the multilayer waveguide loop and active alignment in flip-chipbonding of the pump source.